A co-premise is a

premise A premise or premiss is a true or false statement that helps form the body of an argument, which logically leads to a true or false conclusion. A premise makes a declarative statement about its subject matter which enables a reader to either agre ...

reasoning Reason is the capacity of consciously applying logic by drawing conclusions from new or existing information, with the aim of seeking the truth. It is closely associated with such characteristically human activities as philosophy, science, lang ...

and

informal logic Informal logic encompasses the principles of logic and logical thought outside of a formal setting (characterized by the usage of particular statements). However, the precise definition of "informal logic" is a matter of some dispute. Ralph H. J ...

which is not the main supporting reason for a contention or a

lemma Lemma may refer to: Language and linguistics * Lemma (morphology), the canonical, dictionary or citation form of a word * Lemma (psycholinguistics), a mental abstraction of a word about to be uttered Science and mathematics * Lemma (botany), a ...

, but is logically necessary to ensure the

validity Validity or Valid may refer to: Science/mathematics/statistics: * Validity (logic), a property of a logical argument * Scientific: ** Internal validity, the validity of causal inferences within scientific studies, usually based on experiments ...

of an

argument An argument is a statement or group of statements called premises intended to determine the degree of truth or acceptability of another statement called conclusion. Arguments can be studied from three main perspectives: the logical, the dialect ...

. One premise by itself, or a group of co-premises can form a reason.

Logical structure

Every significant term or phrase appearing in a premise of a simple argument, should also appear in the contention/conclusion or in a co-premise. But this by itself does not guarantee a valid argument, see the fallacy of the undistributed middle for an example of this. Sometimes a co-premise will not be explicitly stated. This type of argument is known as an 'enthymematic' argument, and the co-premise may be referred to as a 'hidden' or an 'unstated' co-premise and will often be subject to an inference objection. In this argument map of a simple argument the two reasons for the main contention are co-premises and not separate reasons for believing the contention to be true. They are both necessary to ensure that the argument as a whole retains logical validity. In this example, "What the Bible says is true" is a hidden co-premise. Informal arguments Statements {{philo-stub